The technology of the separation of plasma from blood by a hollow fiber membrane had firstly been developed for the medical purpose, but the application thereof has nowadays been expanded to the collection of plasma from healthy donors (donorplasmapheresis).
That is, the hollow fiber membrane can be made compact in the volume with keeping the broad area of membrane and can be used in a cross-flow filtration system, and hence, it has widely been used, for instance, in micro filtration, ultrafiltration, in industrial uses such as desalination of sea water or brackish water and ultra-pure water production, and further in medical usage such as purification of blood. Among these usages, a large demand is given to a membrane suitable for microfiltration which is used not only in industrial use such as purification of water in the field of foodstuff, and sterilization of draft beer or wine packed in bottle, but also in medical fields, such as for plasma exchange membrane in patients, and for collection of plasma which is necessary for meeting the increased demand of blood products.
In the treatment of diseases, plasma is separated from blood of patients under observing the state of the patients, and hence, high separation rate is not always required for membrane, but there are required high permeabilities of large molecular weight substances (e.g. immune complex) and stable plasma separation rate (i.e. the separtion rate being constant for a long period of time). Besides, in the case of donorplasmapherisis, the membrane used is required to be able to separate stably the plasma within a very short period of time in order to collect the plasma from healthy donors with little load and pain, and further it is required to have high permeabilities of substances having a large molecular weight like in the case of treatment of diseases. The main object of the donorplasmapheresis is to collect Factor VIII having a large molecular weight which is necessary for the treatment of hemophiliac, and hence, it is the most important that the membrane has a high permeability of the Factor VIII.
Many investigations have been done as to membrane suitable for plasma separation, but available membranes are not satisfactory in view of the stability in the plasma separation rate for the treatment of diseases and also for the collection of plasma and further not satisfactory in the permeability of large molecular weight substances (e.g. Factor VIII, immune complex, etc.).
Besides, a plasma separation module assembled with the plasma separation membrane is to be sterilized. The sterilization is usually carried out by treatment with EOG (ethylene oxide gas), formalin, or .gamma.-ray, or in an autoclave, but in view of efficiency without any residue of the treating agent, the autoclave sterilization is favorable. However, when a separation module assembled with available membrane made of an cellulose ester is sterilized in an autoclave, the membrane is deformed because of less heat stability and the membrane properties can not be maintained.